Expanders adapted to produce power by high-pressure fluid expansion have conventionally included positive displacement expanders, such as rotary expanders (see, for example, Patent Document 1). This type of expander can be used for the execution of an expansion process in a vapor compression refrigeration cycle (see, for example, Patent Document 2).
Such an expander has a cylinder and a piston which orbits inside the cylinder. An actuation chamber, defined between the cylinder and the piston, is divided into two zones, namely a suction/expansion side and a discharge side. With the orbital motion of the piston, the actuation chamber undergoes sequential switching that one zone serving as the suction/expansion side is switched to serve as the discharge side while the other zone serving as the discharge side is switched to serve as the suction/expansion side. In this way, the action of suction/expansion of refrigerant and the action of discharge of refrigerant are simultaneously concurrently achieved.
In the above-described expander, both the angular range of a suction process in which high-pressure refrigerant is supplied into the cylinder during a single revolution of the piston and the angular range of an expansion process in which the refrigerant is expanded are predetermined. In other words, for such a type of expander, the expansion ratio, i.e., the density ratio of suction refrigerant and discharge refrigerant, is generally constant. High-pressure refrigerant is introduced into the cylinder in the angular range of the suction process while on the other hand the refrigerant is expanded at a fixed expansion ratio in the angular range of the remaining expansion process for the recovery of rotational power.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-338356
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-116371